Assemblies of carbon, derived from a variety of carbon sources, have a multitude of current and anticipated commercial, industrial, and high-technology applications. For example, activated charcoal or activated carbon, which is usually in the form of loose powder, particles, or irregular agglomerates, has a variety of uses in filtration and catalyst support. This material has also recently been applied to energy storage applications, as an ionic exchange medium or capacitor electrode material. Graphite in its various forms has numerous uses, for example, as refractory material, in brake linings, and as electrodes in electric arc furnaces. Intercalated graphite and expanded graphite have been studied for use as fire retardants and high temperature applications. These cohesive carbon assemblies have many desirable properties such as resistance to chemical attack, resistance to high temperatures, and high surface area in the case of activated carbon, and electrical conductivity and lubricity in the case of graphite. However, these materials typically require a binder or matrix material to form them into an assembly of a desired shape and size, having good mechanical strength and integrity.
More recently, assemblies of carbon nanotubes (CNTs) in various forms have attracted much attention and are being explored and developed for diverse applications. Such assemblies have been referred to in the literature as “buckypaper” or “buckydiscs”. For example, Dharap et al in “Nanotube film based on single-wall carbon nanotubes for strain sensing”, Nanotechnology 15 (2004), pp. 379-382, investigate the use of isotropic films of randomly oriented CNTs as mechanical strain sensors. Cao et al, in “Random networks and aligned arrays of single-walled carbon nanotubes for electronic device applications,” Nano Research 1, 4 (2008), pp. 259-272, discuss the use of random networks or aligned arrays of CNTs as thin-film transistors. Ma et al, in “Methods of making carbide and oxycarbide containing catalysts,” U.S. Pat. No. 7,576,027 B2, disclose catalyst supports for fluid phase chemical reactions made from randomly entangled CNT aggregates. And Liu et al, in “Electrochemical capacitor with carbon nanotubes,” U.S. Patent Application Publication US 2009/0116171 A1, disclose electrolytic capacitors having electrodes made from free-standing CNT films.
Smalley et al in “Method for producing self-assembled objects comprising single-wall carbon nanotubes and compositions thereof,” U.S. Pat. No. 7,048,999 B2, disclose CNT assemblies formed by a complex process of CNT end-cap removal and derivatization. The buckypaper disclosed therein is a loosely assembled CNT felt or mat that is supported on a substrate. Other structures disclosed therein such as molecular arrays and self-assembled monolayers are described as requiring a substrate or matrix material such as a resin, metal, ceramic, or cermet. Furthermore, the self-assembled structures disclosed therein comprise functional agents to bond the CNTs together, which may adversely affect the structures' electrical properties.
Tohji et al in “Carbon nanotubes aggregate, method for forming same, and biocompatible material,” U.S. Patent Application Publication US 2007/0209093 A1, disclose a method for CNT aggregate formation involving exposure to fluorine gas followed by sintering at high temperature and pressure. The aggregates are characterized as being fragile.
Liu et al in US 2009/0116171 A1, and Hata et al in “Aligned carbon nanotube bulk aggregates, process for production of the same and uses thereof,” U.S. Patent Application Publication US 2009/0272935 A1, disclose methods for preparing CNT assemblies that require the use of CNT forests grown by CVD processes on a substrate. These methods involve a sequence of solvent washing, pressing, and/or drying steps and are limited to the scale of the starting CNT forest. Furthermore, these assemblies are characterized by a predominant orientation or alignment of the CNTs, which imparts the assembly with anisotropic and largely unidirectional properties.
Whitby et al in “Geometric control and tuneable pore size distribution of buckypaper and bucky discs,” Carbon 46 (2008) pp. 949-956, disclose a frit compression method for forming CNT assemblies, which also requires high pressures. Also, the CNTs are not uniformly distributed within the assemblies, and the assemblies have large macropores and very high porosity (>80%).
A method to form a solution of single-walled CNTs in sulfuric super-acids is disclosed by Davis et al in “Phase Behavior and Rheology of SWNTs in Superacids,” Macromolecules 37 (2004) pp. 154-160. A method is also disclosed to produce an entangled mat of CNT ropes by quenching in ether and filtering.
R. Signorelli et al in “High Energy and Power Density Nanotube Ultracapacitor Design, Modeling, Testing and Predicted Performance,” presented at The 19th International Seminar on Double Layer Capacitors and Hybrid Energy Storage Devices (Dec. 7-9, 2009, Deerfield Beach, Fla., USA), and in “Electrochemical Double-Layer Capacitors Using Carbon Nanotube Electrode Structures,” Proceedings of the IEEE 97, 11(2009), pp. 1837-1847, disclose vertically aligned single-walled CNT (SWCNT) and multi-walled CNT (MWCNT) “forest”-type assemblies intended for use as binder-free electrodes. These assemblies, however, show low bulk density of 0.45 g/cm3 or less (0.1 g/cm3 in the case of SWCNT), requiring an impractically high volume of material for adequate capacitor performance. Scalability of these CNT forests for manufacturing purposes is questionable, and they have inferior mechanical properties for use as current collectors.
A similar forest-type assembly produced from double-walled CNT (DWCNT), intended for use as a capacitor electrode, is disclosed by T. Asari in “Electric Double-Layer Capacitor Using Carbon Nanotubes Grown Directly on Aluminum”, presented at ICAC2010, The 2010 International Conference on Advanced Capacitors (May 31-Jun. 2, 2010, Kyoto, Japan). This assembly has similar drawbacks as that of Signorelli; namely, low density, non-scalability, and inferior mechanical properties.
A. Izadi-Najafabadi et al, in “Extracting the Full Potential of Single-Walled Carbon Nanotubes as Durable Supercapacitor Electrodes Operable at 4 V with High Power and Energy Density,” in Advanced Materials, n/a. doi: 10.1002/adma.200904349 (Published on-line Jun. 18, 2010), describe a capacitor electrode based on a high-purity SWCNT forest processed into a binder-free assembly. This assembly shows attractive electronic performance characteristics as an electrode when tested under laboratory conditions. However, a sealed capacitor device could not be produced using this assembly due to excessive swelling when impregnated with the liquid electrolyte, indicating that the assembly had inferior mechanical strength and integrity.
There is interest in applying CNT technology to electrochemical double-layer capacitors (EDLC), sometimes referred to as “supercapacitors” or “ultracapacitors”. This capacitor type has power density somewhat lower than, but nearly approaching, that of standard capacitors, but much higher energy density, approaching that of standard batteries. EDLCs have many applications in consumer electronics, and are attractive for use in hybrid gas-electric vehicles and all-electric vehicles. Activated carbon is the most common material currently used as electrodes in EDLCs. However, its performance may be reaching its technological limit and materials capable of higher energy and power densities are desired, especially for vehicle applications.
Lithium-ion is one battery type of particular interest for application of carbon nanotubes. Modern Li-ion batteries typically comprise a carbon-based anode, a cathode comprising an oxide such as LiCoO2, LiFePO4, LiNiCoAlO2, or the like, and an electrolyte comprising a lithium salt in an organic solvent. Li-ion batteries are commonly used in consumer electronics, and are attractive for use in hybrid gas-electric and all-electric vehicles. However, improvements in battery performance are needed for widespread vehicle application. Specifically, increased energy density, power density, lighter weight, and better reliability are desirable. Particularly attractive are thinner and/or lighter electrode materials having lower electrical resistance, more efficient ion transfer capability, and sufficient mechanical strength for battery use.
In a standard fuel cell, hydrogen is combined with oxygen to generate electric current and water as a by-product. One fuel cell type of current high interest is the proton exchange membrane or polymer electrolyte membrane (PEM) fuel cell. This design comprises a membrane electrode assembly (MEA), which in turn comprises a center proton exchange membrane (PEM), and an electrode on either side of the PEM. Each electrode comprises a catalyst layer and a gas diffusion layer (GDL). The catalyst layer is typically comprised of fine metal particles or powder (platinum for the anode, often nickel for the cathode) on a porous support material such as pressed carbon black. The GDL layer, which contacts the metallic current collector on the face opposite the catalyst layer, is usually comprised of carbon paper or carbon cloth. As in the case of Li-ion batteries, improvements in PEM fuel cell performance are also needed for widespread application, especially in vehicles. Stronger and more lightweight materials, having good electrical conductivity and providing more efficient electrochemical reactions, are desirable for use as electrode materials, as either the catalyst support and/or the GDL.
In various energy storage devices, including capacitors, fuel cells, and batteries, a current collector comprising a metal plate is typically attached to the exposed (outward-facing) surface of the electrode, to collect the current generated by the device and conduct it towards the machine or equipment that the device is powering. Aluminum and copper are typical metals used as current collectors. It is desirable that the weight and complexity of the energy storage devices be reduced, and one such approach is to combine the function of the electrode with that of the current collector in a single material. This may only be accomplished if both the conductivity and mechanical strength and integrity of the material are near enough to those of traditional current collectors, such that the performance of the device is not diminished. In fact, enhancement of the device performance by using a combined electrode/current collector would be ideal.
WO 2010/102250 discloses preparing cohesive carbon assembly by dispersing carbon in a liquid halogen (e.g. bromine), followed by substantial removal of the liquid. However, bromine is highly toxic and expensive. Bromine is also highly corrosive to most metals, significantly reducing its utility as a dispersing medium, since no common metal container or metal surface may come in contact with a dispersion of carbon in a halogen-based liquid.
Therefore, there exists a need for a method for preparing a cohesive carbon assembly that can avoid using the corrosive and toxic halogen solvents.